The little ones will probably get bullied. I plan on waiting with my new ones till they are about a week old before I put them in with the week olds I have now.
They probably should wait till they can get around pretty good, and don't seem weak anymore. A day old chick to older chicks, even a few days old, seem sick, and the others will most likely peck.
I added 2 bantams (1 day old ) and 2 light brahms (1 day old) to a brooder with 6 10 day old chicks. They NEVER pecked at each other. If anything, the older ones kind of looked after the little ones. It was stinkin adorable!
Well I tried it out. I put the newborns in with the others and all are fine. They new ones actually calmed the other ones down a bit. It was odd. The others were very skittish, running and hiding when I opened the brooder, panicking if i tried to grab them. Now they let me touch them and they just stand there. It is much easier to handle them now. Nice!